This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus adapted to illuminate an original document being reproduced therein.
In the process of electrophotographic printing, a photoconductive surface is uniformly charged and exposed to a light image of an original document. Exposure of the photoconductive surface records thereon an electrostatic latent image corresponding to the original document. The electrostatic latent image is then rendered visible by depositing toner particles which adhere electrostatically thereto in image configuration. Subsequently, the toner powder image is transferred to a sheet of support material which may be plain paper or a transparent thermoplastic material, amongst others. The toner powder image is, then, permanently affixed to the support material so as to produce a copy of the original document.
The process of multi-color electrophotographic printing is similar to the process of black and white printing. However, rather than forming a total light image of the original document, the light image is filtered producing a single color light image which is a partial light image of the original document. This single color light image exposes the charged photoconductive surface recording thereon a single color electrostatic latent image. The single color electrostatic latent image is then developed with toner particles of a color complementary to the single color light image. Each single color toner powder image is transferred to the support material in superimposed registration with the prior toner powder image forming thereon a composite multi-layered toner powder image. This multi-color toner powder image is permanently affixed to the support material.
A typical light source may be a tri-phosphor lamp. This type of lamp is arranged to have peak energy outputs at the blue, green and red wave lengths. The corresponding filters are arranged to permit a single color light image to pass therethrough. Hence, a blue filter would only permit the blue light image to be transmitted therethrough, a red filter only a red light image, and a green filter only a green light image. Moreover, it is frequently necessary to advance the lamp along the original document so as to create a flowing light image thereof which is in synchronism with the rotation of the photoconductive drum. In addition, the energy furnished to excite the lamps has to be sufficient to provide minimum brightness for all the various single color images being produced on the photoconductive surface.
The foregoing may be more readily achieved by the employment of solid state light emitters. The usage of light emitting surfaces is disclosed in U.S. Pat. No. 3,438,057 issued in 1969 to Neitzel. This patent discloses a seismic recording system having an elongated solid state light emitting array for exposing a photoconductive film to produce seismic traces thereon.
Heretofore, light emitting diodes have been employed as alpha-numeric display devices. A display device of this type may be electrically coupled to a computer and an electrophotographic printing machine to produce a non-impact printer. This type of system is disclosed by Harris on Page 3,758, Volume 13, No. 12 of the May 1971 IBM Technical Disclosure Bulletin. As shown therein, light emitting diodes are selectively excited by a computer to produce image patterns of print characters. These image patterns illuminate a charged photoconductive drum so as to discharge the photoconductive drum locally in the image pattern of the light emitting diodes. This creates an electrostatic latent image on which a powder image is developed and subsequently transferred to a copy sheet. Thereafter, the powder image is permanently fused to the copy sheet. Other systems of the above-identified type employ cathode ray tubes associated with computers to create illuminated image patterns which discharge the charged photoconductive surface.
However, none of the prior art devices teach the employment of solid state light emitters for creating single color light images or for eliminating the scanning requirement when reproducing transparencies.
Accordingly, it is the primary object of the present invention to improve the exposure system used in an electrophotographic printing machine by the employment of an array of solid state light emitters therein.